Noise suppression circuit



April 7, 1942. L.. KAMENARovxc NOISE SUPPRESSION CIRCUIT Filed Oct. 15, 1938 .l I I l I I I I lI '-l l.

INVENTOR. LA v KA MEA/Ano wc' ATTORNEY.

Patented Apr. 7, 1942 Umso smrss ATENT OFFICE NOISE SUPPRESSION CIRCUIT Wavre Application October 15, 1938, Serial No. 235,151 In Italy October 29, 1937 (Cl. Z50-20) 3 Claims.

Radio receiver apparatus comprising means adapted to the automatic control of the volume or sensitivity and their operation involve the problem of suppressing the reproduction of noises in the intervals between the stations or the carrier waves by what is known in the art as silent tuning.

As a matter of fact, the automatic volume control means tend to stabilize the level with which the signals act upon the receiver and as a result modify the sensitiveness or responsiveness of the receiver set in inverse proportion to the intensity of the incoming signals. Therefore, during the intervals between the various stations or carriers the automatic volume control acts in the sense of raising lthe sensitivity to the maximum value and as a result raises unduly and excessively also the noise level so that the receiver set works very noisily while iishing for or tuning in the various transmitter stations.

To the end of avoiding these diiculties silent tuning means have been disclosed in the prior art which are adapted to render inactive or inoperative the receiver set whenever the incoming signals fall below a predetermined level, and this is designed to eliminate to a large part the disturbing actions or noises the level of which as a general rule is fairly low, and to insure the tuning of transmitter stations in the presence of normal volume and under fairly noiseless conditions. However, the ways and means disclosed in the art to this end have fallen short of vfulfilling requirements; in fact, when a station which is to be received is subject to uctuations, for instance, as a result of fading, about the level at which the silent tuning means is caused to become operative, reception will be periodically interrupted and restored at full volume, while if the signal is maintained for a certain length of time at the stated level, the reception or reproduction turns out to be seriously distorted on the ground that the circuit is in a critical operating state.

The before-mentioned inconvenience can not be eliminated simply by fixing at a lower point the level where the silencing means is caused to enter into operation nor can this be achieved by making the said level regulable or variable, because in the rst case it is not possible satisfactorily to eliminate the noise while attempting to tune in a station, whereas in the second case, if regulation is fixed at a level high enough to insure silence or freedom from noise in the intervals between carrier waves, there.. is a risk of mutilating signals subject to fading even when not very strong.

The present invention has as its object a silent, tuning or a noise suppressing means which is adapted to obviate the above-mentioned difculties. In fact, by the aid of the same it is possible to make conditions so that when the silent-l tuning means is rendered inactive at a given signal level, the same is reactivated or restored to operating condition only when the signal falls to a level far below'the level where inactivation occurs. Hence, once reception of a given station or signal has been established, such reception is maintained, subject to the action of the automatic Volume control means, even when such station signal experiences an appreciable weakening 0r fading. On the contrary,`

when tuning the receiver set to another station, the very strong drop in input which at all events happens in the interval between carrier waves insures operation of the silencing or noise suppressing means.

The appended drawing Fig. 1 schematically represents an arrangement predicated upon the invention as applied to a receiver set of which are shown merely such elements as are strictly indispensable in connection with the present invention. Fig. 2 is a schematic view of a modification of part of the arrangement. Figs. 3 and 4 are graphs adapted to explain the behavior and operation of the arrangement here disclosed.

Referring to the key diagram Fig. 1, A is an I. F. amplifier upon the grid of which are brought to act the signals coming from the frequencyv converter circuit CF in the case of a superheterodyne receiver, which, in turn, is excited from the antenna circuit. The said valve A could also form a part of the last RF stage where a tuned RF type of receiver is concerned, the plate of the said valve or tube being connected with the tuned circuit I, and across a capacitor cs with the oscillating circuit 2.

The oscillatory circuit I inductively excites an oscillatory circuit 3. To the terminals of the capacity of the latter are connected the plate and across capacitor c1 the cathode of a diode D1 while connected with oscillatory circuit 2 in a similar manner are the anode and the cathode of adiode Dz, with a capacitor c2 being interposed in the cathode lead.

the anode of a silencing valve S which is also connected-with the circuit of the grid of valve A across the resistances 6 and 6', which, conjointly with capacitor cs constitutes a lter adapted to level or smooth the biasing potential,

. while conductor 4 is connected with a terminal of a resistance R2 whose opposite terminal is united with the cathode of diode D1. Fitted on resistance R2 is an adjustable tap or slide 'I to vwhich is connected a conductor 8 which forms the terminal of and is brought to the AF circuit "(BF) of the receiver. y

The oscillatory circuit 2 is connected with ground through 9 and the resistance R1. The terminal of the said resistance R1 which is united with circuit 2 is also connected through I8 withv the grid of the silencing tube S across the resistance R3; the opposite terminals of the latter are grounded by way of capacitors c4 and es, respectively.

The cathodes of valve A and of diode D1 are connected by means of leads I 0 and II with a movable slide or tap I3 on a resistance I4 having one terminaly connected with a positive source ofV supply, while the opposite terminal is grounded. The cathode of diode D2 is similarly connected by Way of conductor I5 and the slide I6 with a resistance Il connected between a positive source of supply and theground.

Supposing a signal is received at a level adapted for normal reception subject to the action of the automatic volume control means, if taps I3 and I6 arelplacedon the ground-end terminals of the respective resistances I4l and I1-, the silencing means isprevented from becoming operative. In fact, detection insured byA diode D1 gives rise to a. negative potential across the terminals of re-v sistance R2, and the anode of silencing valve S in the art, by'. virtue of amodication of the biasing 'of the grid of tube A as afunction of/ the strength of theincomingrand detected signal.

Rectification in diode D2 of the said incoming signalimpresses through 9 and` I8 a negative bias uponrthe grid of valve S thereby assisting in maintaining thefsame inoperative.

,Supposingthat taps or slides I 3 and I6 are shifted away from the ground-end terminals of resistance I 4. and I 1, there will act on the said taps and their respectivel conductors I 0l,-A I I and a I5 positive potentials hereinafter to be called V1 and V2. The positive potential V1 is applied to the. cathode of tube A and to the cathode of diode D1 and acrossR2 to` the-ranodel-of valveS whose internal drop, in theV absence of a signal, may' bev considered negligible incomparison with the resistance R2 because under these conditions its grid is connectedwith ground so that the valve presents a low inner resistance for direct current. Consequently, the voltage V1 applied to the cathode. of diode D1 results in a D.-C. which is made through ground across R2 and S.- The D. C. po-

tential corresponding to the fall caused by this current across the terminals of the resistance R2 is impressed through 6 and 5Y upon the grid of amplifier valve Aand'furnishesfthe initial-bias@ ing actionwhich reduces theamplication or gaini thereof. Thesame dropon:the other hand, acts as apositivebias uponr diode D1 andA thus main tains2the cathode ,of diode Di, more f positiver thanthe anode and prevents the detection of alternath ing currents of the signal fed from valve A to diode D1 by way of circuits I and 3 having an intensity or strength less than that corresponding to the said biasing action.

Similarly, the biasing of diode D2 insured by potential V2 prevents detector action in the said diode of signal currents fed thereto from tube A across C3 and 2, c2 as long as such signal currents do not surpass the value corresponding to biasing voltage V2.

The receiver therefor is silent inasmuch as detector action in D1 is precluded, and the responsiveness thereof isA diminished by virtue of the action of potential V1 upon the grid of tube A. The receiver stays silent as a result of the cut- .off of diode D1 even if the intensity of the signal grows' to a certain extent.

When the signal reaches a value (s1) so as to result in 2 in an alternating tension Vs sufficiently high to overcome the positive biasing action produced by potentialLV2 upon diode D2, detection of the signal will be brought about by the said diode and there arises a negative potential across the terminals of R1 and therefore upon the grid of tube S.

In the presence of conditions as described, the inner resistance of valve S for D. C. increases; and asa resultthere occurs a decrease in the current which ows'through resistance R2 as a result of potentialV1 andthe drop of potential across the terminals of R2. AsA a result the biasing potential acting upon: the grid of tube A due to V1 decreases, thereby resulting in agrowth in gain of this valve, which, in turn, is attended with a growth in: signal strength acting upon diode D2. This increaseinsignal strength, in turn, raises still further thenegative bias. acting upon the grid of S andathe inner resistance of this valve Witha further reduction ofthe current due to V1 iiowing. in resistance R2 and the bias action upon the grid of tubeA so that thel gain of this valve thereafter iscaused to grow.

The inner resistance ofthe silencing valve- S therefore tends instantaneously to assume the maximum; value permitted by and-corresponding to the circuit constants and the intensity of thel incoming signal,. thereby resulting inthe suppression of the biasing action due to V1 upon diode D1 andupon thegrid' of tube A, With the consequence that-,the diode: D1 insures normal detector action. to` excite the AF circuit, whereas the D., C. component resulting from the detector action in diode-D1 produces the normal AVC action` upon valve A; in the presence of which, on the contrary, silencingvalve Slisno longer operative. Thisoperation which is the'A same as thatf already described inthe absence of voltage V1I subsists for th'eentire lengthxof time. for which theincoming signals inhere suiiicientintensity to result in D2 a potentialzVsfcapable of overcoming the action of constantpotential V2, taking intoconsideration the gain established in valveA by' action of the same diode D2 with exclusion of theinitial bias potential across the terminals of R2.

Taking into consideration/that the action of' diode D2 in thefsense.of.cutting oithe silencingi valve S is-initiatedwhen the signal potential Vsactingl upon diode D2is capable of vsurmounting potential V2 While valve A operates at-the low gain'corresponding to the total drop produced by4 V1 across resistance R2, it evidently follows that; theconditionsadapted to activate or render opn erative the valveS` so' as to'maketh'e" receiver set` 1 silent-are` able`4v to beI restored again' only when' the intensity of the signal falls below a value su which is appreciably lower than s1.

. Conditions are therefore made so that during intervals between stations or carriers the receiver apparatus will stay perfectly silent so far as disturbing actions are concerned even when these reach an appreciable level, once that normal operation has been established at a level adequate to produce the cut-off of the silencing valve, and this operating state subsists also in the presence of the possible condition where the station signals suffer considerable fading, though there is, on the contrary, a tendency of such fading being compensated by virtue of the automatic volume control. In practice it is possible to make the ratio between the signal voltage (s1) corresponding to the cut-off of the silencing valve and the potential of the signal (so) corresponding to the activation or operation of the said valve equal to 500, it being thus possible to preclude all disturbances and interference during inter-station intervals and permit very appreciable losses of the signal in reception without the receiver vapparatus being rendered silent.

tivation are produced which correspond to the l before-mentioned values S and s of the signal.

As a matter of fact, by shifting slide I6 away from the ground-end terminal of resistance I'I, the potential V2 fed to diode D2 is raised with a resultant raise of the Values of the signal potentials (VS1 and Vso) in the presence of which the cutoff and the opening or unblocking of the silencing tube takes place. If, on the contrary, slide I3 of resistance I4 is moved in a direction away from the ground-end terminal thereof, potential V1 is raised, and this means an increase in the initial biasing of the grid of tube A in the sense of lower gain, thereby raising the level of the incoming signal potential (s1) for which the silencing valve S is cut off without an incidental change in the level of the incoming signal potential (so) in the presence of which the said valve is opened or re-activated. Hence, by the combined adjustment of taps or slides I6 and I3 it is possible to change independently of each other the points where the silencing valve is Opened and Where it is cut 01T.

Fig. 2 illustrates regulating means adaptable to the circuit organization hereinbefore disclosed by substituting for the assembly Z in Fig. 1 comprising elements I3, I4, I6, I 7, the assembly of regulator means Z shown in Fig. 2, while the rest of the organization shown in Fig. 1 remains unchanged.

In this case, the conductors II and I which are brought to the cathodes of diodes D1 and D2 are connected jointly to a common point of a resistance I9, one terminal of which is grounded, while the other terminal thereof is united with the conductor I0 which is brought to the cathode of tube A. Between the said cathode and the resistance IB is shunted to ground a variable resistance R4.

The potential across resistance R4 corresponds to the fall produced by the filament current of amplifier tube A. Potentials V1 and V2 are here replaced by one potential arising across the terminals of portion Re of resistance I9 included between the variable tap and ground, whereas section R5 stays permanently included between the lament and the grid of valve A, thereby resulting in a permanent biasing action, which persists also after cut-off of valve S, to regulate the gain of valve A.

' By regulating the value of R4 there results a corresponding change of the potential across the terminals of Rs and this results in a change both in the point Where the valve S is unblocked as well as the point where it is cut off; there is changed at the same time also the potential across the portion designated by Rs and thus also the effective responsiveness or sensitiveness of the valve A after Valve S has been cut oi.

Changing, nally, the ratio between sections R5 and Re it is possible to insure a far wider regulation of the range between the points Where silencing tube S is opened and where it is cut oi.

This regulation means offers the advantage that it is possibleto use a double diode tube comprising a joint filament or cathode since the leads I I and I5 may be combined in a single conductor; that additional sources of supply of potential are dispensable, and that, nally, it is possible to regulate simultaneously the cut-off point of the silencing valve and the gain of tube A and thus the effective sensitiveness of the receiver set in its normal operation after the silencing tube has been cut off.

Fig. 3 is shown to explain more clearly the behavior and operation of circuit organizations of the kind hereinbefore disclosed. Referring to Fig. 3 the abscissae stand for incoming signal voltages (s) while the ordinates stand for the values of the potentials acting upon D2 and thus the D. C. potential V2 applied to the cathode of the said diode and the amplitude of the potential Vs of the amplified signal acting between the cathode and the anode of the said diode. Assuming that the receiver is in silent state, plotting the incoming signal from O to the value so (abscissa), the alternating potential Vs acting on V2 grows linearly and to a limited extent since the diode D1 is maintained inactive so that the AVC is inoperative, while, on the other hand, upon the grid of tube A there acts the bias corresponding to the fall produced by the current which ows through the resistance R2 and valve S being still active by virtue of the potential V1 applied there- When the incoming signal has a value s1 and the potential impressed upon diode D2 reaches a value VS1 equal to V2 the operation is such as described, that is, diode D1 becomes operative and there occurs an increase in gain (as represented by the ratio of the ordinate and abscissa of the straight graph VS and thus by the tangent of the angle between VS and the abscissa axis) from the value a1 to value a2 as a result of the action as described upon the grid of valve A. Potential.

VS1 acting between the anode and the cathode of diode D2, by virtue and as a consequence of variation of amplification or gain, changes instantaneously to value VS2. If, then, the value of the incoming potential s diminishes, the gain preserves its higher value a2, the potential Vs upon the diode D2 is not restored to the value VS1, but decreases in accordance with curve VS2-VSO, the trend or shape of the curve being also a function of the automatic volume control. along this curve the potential upon D2 decreasing from the value VS2 reaches value Vso equal to V2, there is brought about the operation or opening of v alve S, and this makes the receiver set silent again.

If, by varying.

Setting potential V2 at another value, say, V'z, there results a similar behavior as represented by points Vsi, Vsz and Vs'o.

In other words, potential Vs follows the shape of the curve O--Vsi--Vsz as the incoming signal potential grows from O to s1, while the same varies according to the curve Vsz-Vso-Vsoo and eventually O (if the incoming signal diminishes still further) while the incoming signal potential varies from s1 to so (and to O).

Fig. 4 illustrates the corresponding trend and shape of the A. F. incoming potential (ordinate:t(bf)) plotted against the incoming signal potential .(abscissa:s). The A. F. incoming potential is O during the intervals where the incoming signal potential grows from) to si whereas the same varies according to the curve Tsi-Tsz-Tso-O while there occur variations in decreasing sense of the incoming signal potentials from value si to value so.

What is claimed is:

1. In a radio receiver of the type comprising a signal amplier followed by a detector having a load impedance across which is developed detect: ed signal voltage, the improvement which comprises a direct current voltage connection from the load impedance to said amplier for gain control of the latter, an electron discharge device including said load impedance in its space current path and. normally developing across the impedance a direct current voltage applied to said detector in detection-suppression sense, a diode rectifier for deriving a direct current voltage from amplified signals, means for applying the derived voltage to said electron discharge device in a sense to prevent development of said suppression voltage, and variable bias means in circuit between the diode anode and cathode for rendering said diode rectifier ineffective in the absence of signals whose amplitude exceed a predetermined magnitude.

2. In a radio receiver of the type comprising a signal amplier followed by a diode detector having a load impedance across which is developed detected signal voltage, the improvement which comprises a direct current voltage connection from the load impedance to said amplifier for gain control of the latter, an electron discharge device including said load impedance in its space current path and normally developing across the impedance a direct current voltage applied to said detector in detection-suppression sense, a second diode for deriving a direct current voltage from amplified signals, means for applying the derived voltage to said electron discharge device in a sense to prevent development of said suppression voltage, variable bias means in circuit between the second diode anode and cathode for rendering said second diode ineffective in the absence of signals whose amplitude exceed a predetermined magnitude, and means for rendering the said electron discharge device conductive in the absence of signals of a predetermined amplitude and the electrodes of said two diodes being in a common tube envelope.

` 3. In combination with the signal amplier of a radio receiver, a pair of diodes, means for impressing signals upon one diode from said amplifier, a load resistor in circuit with said one diode for developing audio voltage from the rectified signals, an automatic gain control connection from the resistor to said ampliiier, means for concurrently adjusting the cathode potentials of each of said amplier and one diode, means for impressing signals upon the second diode from said amplifier, an impedance in circuit with the second diode for developing a unidirectional voltage from rectified signals, means for separately adjusting the second diode cathode potential, an electron discharge tube including said load resistor in its space current path whereby space current flow through the load resistor develops a unidirectional voltage, said latter voltage being applied to said one diode in rectificationsuppression sense, and means for applying the second diode voltage to said electron discharge tube in a sense to reduce its space current flow.

LAV KAMENAROVIC. 

